1. Field of the Invention
The present invention relates to a control system for a furnace used to heat metal workpieces in a generally continuous manner to a predetermined furnace discharge temperature incident to processing of the metal workpiece, and more particulary to a method and apparatus providing a single-point temperature control for a multi-section workpiece heating furnace to heat a metal workpiece continuously traveling through the furnace without adverse influences arising out of changing dimensions to the thickness of the workpiece being processed.
2. Description of Related Art
It is well known in the art to continuously advance a metal workpiece in a furnace having a plurality heating chambers to heat the metal workpiece to a desired furnace discharge temperature for achieving an intended thermal treatment which can be part of a more extensive metal working processes, for example, for heat treating, annealing and galvanizing processes. Furnaces for such processes are usually in the form of a tandem arranged sections. The initial heating stage serves to clean by combustion of contaminants on the surface of the workpiece and thereafter additional heating stages continue heating of the metal workpiece so that the workpiece is discharged from the furnace at a desired or predetermined temperature.
The metal workpiece may be wire or any of a variety of structural shapes, coiled strip, and metal sheets. In a continuous annealing and galvanizing lines, a metal strip is processed in an endless fashion by supplying metal strip from discrete coils having the terminal end of one coil welding or secured to the trailing end of a next coil to be processed. A furnace for heating such a continuous strip when made up of multi-section furnace sections, is typically made up of at least three furnace sections that function to heat the strip in a first furnace section sufficient to clean contaminants from the strip. A second furnace section further heats the strip to a temperature below the desired furnace discharge temperature which is subsequently obtained by further heating in a subsequent, third furnace section. An example of such a multi section heating furnace is found in Japanese Patent Publication 57-19336 laid open Feb. 1, 1982 for a continuous annealing facility in which the strip is supplied to a gas heating zone followed by an induction heating zone and then supplied to a soaking zone. Temperature detections are provided before and after the induction heating zone for supplying signals to a temperature controller. The controller also receives reference value of target strip temperature, welding point, line speed, thickness and width.
Temperature sensors, such as infrared and optical pyrometers, are used to measure the temperature of the strip at each of multiple locations along the furnace in continuous processing line. A temperature sensor is typically located at the entrance or exit of a furnace section in a multi section furnace. The workpiece temperature measurements are then used to adjust the temperatures in the furnace section to achieve a desired workpiece discharge temperature.
In one prior art galvanealing furnace with a fossil-fueled, direct-fired first furnace section, followed by an electric induction heating coil second furnace section, in turn, followed by a fossil-fueled, radiant tube heating section, has at least one temperature sensor located in each of the three furnace sections. The outputs of the temperature sensors are entered to a computer system, along with the width and thickness of the workpiece. A heating algorithm is executed within the computer to command the power outputs of the first, second and third furnace sections. The computer control system used to modify the operation of all three furnace sections includes a temperature sensor associated with each furnace section. The control system produces changes to the operation by all three furnace sections and is therefore believed unduly complex to such an extent that the control system could become unstable due to continuous corrections particularly by the third furnace section. The instability could be greater by additionally involving the second furnace section due to a cascading effect introduced by changes at the first furnace section either by the control or changes to thickness or other properties of the strip.
It is an object of the present invention to provide a rapidly responsive single-point temperature control system to maintain a predetermined workpiece temperature at a predetermined measurement site in a control furnace section which adjusts for temperature deviations to an incoming workpiece by a preceding furnace section to assure the delivery of a workpiece with a predetermined temperature from a following furnace section.
It is another object of the present invention to establish by use of a mathematical expression a desired temperature set point in a second furnace section of a tandemly arranged first, second and third furnace sections for control of the second furnace section for delivering workpieces from a third furnace section at a predetermined desired temperature.
It is another object of the present invention to maintain a desired exit furnace temperature for a workpiece by maintaining a constant temperature at a predetermined fixed point in a second furnace section at a location dependent upon the active lengths of three furnace sections and the average specific heating powers of the first and third furnace sections of the furnace.
It is a further object of the present invention to maintain a desired workpiece discharge temperature from a furnace by maintaining a fixed measurement point temperature at a site along a second of three tandemly arranged furnace sections using a control dependent upon the required output temperature of the workpiece, the temperature of the workpiece at the entrance to the furnace, the active lengths of the first and third furnace sections, and the average specific heating powers of the first and third heating sections of the furnace.